Your linked article had this line "It’s unlikely that Netflix and company will flip the switch for Linux Firefox users right away, so some short-term user-agent switching may be necessary (as it was on Chrome to begin with)."
– AnwarSep 22 '16 at 14:48

I can't even get Netflix to play on Firefox on Windows, which the article claims is already possible, so I think Netflix is currently only allowing it for Chrome regardless of Firefox's technical support for it. Changing the user agent as stated below seems like the only way.
– Aaron FrankeSep 22 '16 at 15:32

1

In case someone is still stuck on this in 2020. I was missing : sudo apt install libavcodec-extra This command solved my issue, I am using Ubuntu 20.04 with Firefox 71.
– Varad BhatnagarJun 2 at 19:44

Thanks, that worked. I'm going to edit your answer a bit to include all the steps I had to do.
– Organic MarbleSep 22 '16 at 15:29

2

@TenLeftFingers from a quick search you could try this extension to override a user agent only for Netflix.
– Hi-AngelSep 23 '16 at 16:23

2

Props to Amazon Video for this working immediately upon the upgrade to Firefox 49. Upon starting video on Amazon, you just get prompted to install Widevine, which will of course work with Netflix after the above User Agent spoofing.
– CanutesonSep 26 '16 at 3:20